Thunderbird uses Bayesian filtering to classify junk mail, which is a system that requires some degree of user intervention and training (see the FAQ on the next page for an explanation of how Bayesian filtering works). Steve Ballmer, the CEO of Microsoft, was quoted in the same story as saying that an entire department at Microsoft is devoted to doing nothing more than ensuring that nothing unwanted gets into Gates' inbox. Luckily, Thunderbird has an excellent way to keep spam in check. Ready to enter a contest that has a prize that looks too good to be true? It just might be that the contest you are entering will lead you down the primrose path to an inbox full of spam (not surprisingly, the entry form probably only asked for your email address). If you put your email address out in the Internet space, it is likely at some point that your address will be harvested by spammers and you will become a victim of spam email. At least I don't get as much as Bill Gates, who reportedly receives four million emails a day, most of which are spam.

Junk, junk, and more junkit seems that some days I get more spam email than I do legitimate email.

How to Train Thunderbird's Junk Mail Filter

Mozilla thunderbird mail filter